The present invention relates to a dielectric drum and an electrostatic recording device using the dielectric drum and, in particular, to a dielectric drum including an aluminum substrate and a dielectric layer of an anodic oxidized film disposed on the surface of the aluminum substrate as well as an electrostatic recording device which uses such dielectric drum to transfer a toner image by pressure or to transfer and fix a toner image by pressure.
As a conventional electrostatic recording device, for example, there is known a device which is disclosed in Unexamined Japanese Patent Publication Sho. 60-500831. In this electrostatic recording device, an electrostatic latent image is formed to an image signal on a dielectric drum properly by use of a latent image forming device, the electric latent image on the dielectric drum is actualized by means of a toner contained in a developing device, and after then the toner image on the dielectric drum is transferred by pressure or transferred and fixed by pressure onto a medium to be transferred by use of a pressure roll.
A dielectric drum, which can be used in such electrostatic recording device, must be formed of a material which is light in weight in order to satisfy a condition that the electrostatic recording device must be light. Also, the electrostatic recording device must have a hardness and a tensile strength which are sufficient to withstand mechanical stresses produced in a pressure transfer step or in a pressure transfer/fix step. Further, the recording device must have a hardness which is sufficient to avoid a collapsing phenomenon which could occur on the surface of the dielectric drum due to generation of a wrinkle in a recording paper. In other words, if a wrinkle is formed in the recording paper, then high pressure points form locally on the surface of the dielectric drum, so that the portions of the dielectric drum surface corresponding to the wrinkles in the recording paper collapse down. This is referred to as "collapsing phenomenon" in this specification. The collapsing phenomenon induces poor transfer, poor cleaning or the like which results in a poor image quality and reduces the life of the dielectric drum itself.
In view of this, conventionally, a dielectric drum is manufactured by performing an anodic oxidizing processing on the surface of an aluminum substrate formed of an aluminum alloy pipe of the 6000 or 7000 series alloys of the Aluminum Association and then by disposing a dielectric layer of an anodic oxidized film on the surface of the aluminum substrate. In addition, in order to keep an image transfer property, a cleaning property or the like, the surface roughness (here, the maximum height [JIS]: the greatest amplitude in a standard length) of the dielectric drum must be, for example, 0.5S (maximum height 0.5 .mu.m) or less at the time when the aluminum substrate is finally produced.
However, when a pipe is formed of the 6000 or 7000 series alloys of the Aluminum Association (especially, the 7000 series is referred to as a hard metal) having high hardness and tensile strength, it is not easy to work the pipe by drawing, resulting in the increased working costs thereof. In addition to the high costs, even if an impurity control processing is performed on these aluminum alloys (for example, a homogenization processing by heating which is performed while the aluminum alloy is in a billet state before the draw working of the pipe), because such aluminum alloy contains various kinds of impurities, a large number of under-surface voids and surface pits occur in an anodic oxidization treated film, which voids and pits are unfavorable because they deteriorate the property of the film.
Also, the aluminum alloy of AA6000 or 7000 series Alloy having high hardness and tensile strength generally contains a large number of impurities (metals) and the surface roughness thereof is increased when it is anodically oxidized. Therefore, normally, in homogenizing the dielectric property of the anodic oxidized film, the surface of the pipe formed of such aluminum alloy is finished after the anodic oxidization treatment so as to be able to keep the above-mentioned surface roughness within an allowable range. In this surface finishing, due to the fact that the anodic oxidized film has a surface hardness of the order of 5 times that of the aluminum alloy, it takes a long time perform a surface finishing step after the anodic oxidizing treatment, which makes it difficult to obtain an anodic oxidized film having a uniform dielectric property.
In other words, when the 6000 or 7000 series alloy is used as a substrate for a dielectric drum which includes a dielectric layer of an anodic oxidized film, the manufacturing costs of the dielectric drum are high and also there arises a technical problem that it is difficult to obtain an anodic oxidized film having a uniform dielectric property. As a result of this, an electrostatic recording device using this kind of dielectric drum is inevitably expensive and there arises a possibility that a reliability on the dielectric property of the dielectric drum is to be degraded.